I am making a docker setup for a class on microservices I am taking. Can someone please tell me why the frontend container cannot make requests to the Flask server running on the user container through it’s alias? The aliasing does seems to work between the connection of the user and user-db containers, so why not between the user and frontend containers?

docker-compose.yaml

services:
  # frontend service
  frontend:
    container_name: frontend
    build: ./gui
    # Specify internal network
    networks:
      host:
      user-service:
    # Expose the frontend service on port 5000
    ports:
      - "5000:5000"

  # User service
  user:
    container_name: user
    build: ./user
    # Specify internal network
    networks:
      user-service:
        aliases: [ "user" ]
      user-db:
    # Wait for the user-db service to be healthy before starting the user service
    depends_on:
      user-db:
        condition: service_healthy

    # User service database
  user-db:
    container_name: user-db
    image: postgres:16.3-bookworm
    # Specify volume
    volumes:
      - user-db-data:/var/lib/postgresql/data
    # Specify internal network
    networks:
      user-db:
        aliases: [ "user-db" ]
    # Set environment variables
    environment:
      POSTGRES_USER: root
      POSTGRES_PASSWORD: glowing-banana
      POSTGRES_DB: user_db
    # Check for correct startup
    healthcheck:
      test: "pg_isready -U 'root' -d 'user_db' || exit 1"
      interval: 10s
      timeout: 10s
      retries: 5

volumes:
  # Define volumes to ensure data persistence of service databases
  user-db-data: {}

networks:
  host: {}
  # Define networks to be used internally
  user-service:
    internal: true
  user-db:
    internal: true

code in backend

from flask import Flask, request, Response
from flask_restful import Api, Resource

app = Flask("User")
api = Api(app)

class Test(Resource):
    def get(self) -> Response:
        return Response("Hello World!", status=200)

api.add_resource(Test, "/")

if __name__ == "__main__":
    app.run(debug=True)

code in frontend

response = requests.get('http://user:5001/')

Because this is a project for a class on microservices, exposing all containers and having them communicate through localhost is not an optimal solution, altough it would work. I am looking for a way to make this work using internal networks.